1. Field of the Invention
The present invention relates to modulation of radiation in optical fibers. More specifically, the present invention relates to production of a sinusoidally modulated laser beam by cross phase modulation of light in an optical fiber.
2. Description of Related Art
Laser radiation has application in a wide variety of disciplines, such as communications, medicine, the military, research, and any other field where directed electromagnetic radiation is an advantage. The light produced from a laser has many known applications, and it is reasonable to expect that many applications of the laser have yet to be discovered.
Some laser uses require phase modulation of the laser beam for efficient operation. As an example, high power lasers, which may comprise a number of lasers or power amplifiers connected together, can benefit from a phase modulated laser beam. In typical high power lasers, a "seed", or "master" oscillator generates a laser output which is provided to one or more power amplifiers. This type of configuration may be termed a "MOPA" (Master Oscillator-Power Amplifier) configuration. The seed oscillator may provide a coherent beam (constant phase) of collimated light, or it may provide an incoherent beam (random phase) to the following power amplifiers.
If a high degree of coherence of the output beam is not required, incoherent light within the laser system may be advantageous. Since totally incoherent light has a random spatial and temporal phase, there are no diffraction or interference patterns formed. As a result, there would be no areas where the localized intensity may vary greatly from the average intensity, and thus the laser could operate close to its damage threshold. Phase modulation can be useful in creating an incoherent beam.
A common type of phase modulation is a simple periodic sinusoidal modulation. This type is common because it can be accomplished using simpler hardware than other types of modulation. To create simple sinusoidal modulation, the modulating element may be an electro-optic crystal driven by a sinusoidal RF voltage. The electro-optic crystal changes its index of refraction according to the applied voltage. However, large bandwidths and high modulation frequencies are difficult to accomplish with an electro-optic crystal due to the limitations of electronic RF generators. Expensive, high power microwave generators have been built for this purpose; however, the electronics acts as a basic limit on the modulation frequency and the amount of bandwidth expansion that can be obtained using electro-optic crystals.
A laser beam that has been phase modulated has a broader bandwidth. A wide bandwidth means a wide variation from the pulse's center wavelength. A laser beam with a broad bandwidth may also be useful for extracting energy from inhomogeneously broadened amplifier media, where different wavelengths are amplified by different excited atoms. Extracting energy from more of the atoms can result in higher amplifier efficiency.
As mentioned above, phase modulation can be useful in creating an incoherent beam. Phase modulation itself imparts a certain bandwidth to a beam, but the beam is still spatially coherent. Various systems have been developed at major laboratories for converting a pulse of spatially coherent light into a pulse of incoherent light. These systems require that the pulse have a certain finite bandwidth for optimal incoherence conversion. Each system has different bandwidth requirements. For example, a system developed at the University of Rochester--Smoothing by Spectral Dispersion (SSD)--requires a 2 .ANG. to 4 .ANG. bandwidth (FWHM), while a system developed at the Naval Research Labs--Induced Spatial Incoherence (ISI)--requires a 20 .ANG. to 30 .ANG. bandwidth (FWHM). The ISI system is described in an article by R. H. Lehmberg and S. P. Obenschain, "Use of Induced Spatial Incoherence for Uniform Illumination of Laser Fusion Targets", Optics Communications, Vol. 46, No. 1, pp. 27-31, Jun. 1, 1983, and in another article by Lehmberg et al., "Theory of Induced Spatial Incoherence", J. Appl. Phys., Vol. 62, No. 7, pp. 2680-2701, Oct. 1, 1987. The SSD system is described in an article by Skupsky et al., "Improved Laser-Beam Uniformity Using the Angular Dispersion of Frequency-Modulated Light", J. Appl. Phys., Vol. 66, No. 8, pp. 3456-3462, Oct. 15, 1989.
Significant applications exist for high power incoherent light. One such application is inertial confinement fusion, where total incoherence of laser light on target may provide significantly improved efficiency in coupling the beam to the target. Such an application requires that the output laser beam or beams be both spatially and temporally incoherent. In practice, temporal incoherence alone can be obtained by amplifying broadband laser light with a laser amplifier chain.
In a laser system each material through which the laser passes has a damage threshold which describes the peak electric field amplitude or intensity of the laser pulse that can pass through the system without damage to the components. Average power output from the laser is severely limited by rapid intensity fluctuations. Therefore, to avoid damage to the components while increasing average output power, time fluctuations in intensity should be minimized. Minimizing (i.e., smoothing) the peak intensity fluctuations of a temporally incoherent pulse over time can provide a high average power to the target, because the pulse can propogate through the system at an average intensity just below the damage threshold.
Spatial incoherence of the light may be required for certain target irradiation experiments related to laser-driven inertial confinement fusion (ICF) of deuterium and deuterium/tritium filled spherical target shells. It is now understood in the ICF field that laser irradiation non-uniformity on targets must be less than a 1% root-mean-squared (rms) deviation from the average intensity over the target surface. Focussed radiation from today's solid-state or gas laser systems can not achieve this degree of intensity uniformity on target. In addition, local laser radiation "hot-spots" on target cause many undesirable light scattering instabilities in the under-dense coronal plasma surrounding the target sphere. These plasma instabilities cause severe scattering of the incoming laser light away from the target, causing further radiation and plasma nonuniformities, thereby preventing target compression and nuclear fusion.
It would be advantageous to have a laser system that can provide a specific bandwidth reliably and conveniently while maintaining a smooth temporal pulse shape. It would be a further advantage if the bandwidth could be widened in an amount beyond that obtainable by electro-optic crystal modulation. For research using these and other methods, it is desirable to be able to conveniently and continuously vary the bandwidth of the pulse.
For other applications, such as laser pulse compression, it is desirable to have a pulse that has a broad bandwidth while retaining a single temporal and spatial mode. It is advantageous if the pulse has a spectral content that is approximately evenly distributed around the central wavelength.
For some uses, a temporally smooth laser pulse is an advantage. A pulse is temporally smooth if it has an intensity as a function of time that does not change abruptly; i.e., the pulse has an average intensity that is close to its peak intensity. Thus, it would be a further advantage if a temporally smooth laser pulse can be broadened in wavelength while retaining its temporally smooth shape, for safe and predictable laser operation at high power.